html: output both @classdesc and constructor

Documentation for a class should include both the class-level description (from @classdesc) and the description for the constructor.

Help wanted summary

• Intent: To show the classdesc tag in HTML output. Here's the documentation for classdesc on JSDoc's site.
• Requirements: This task requires JavaScript skills, but can be done without deep changes to documentation.js.
• Where: this lodash template in default_theme is where we output section.description. You will add a new section ( here are examples of template syntax ) that will check whether section has a classdesc property, and if it does, output it onto the page. The value of the classdesc property will be a Markdown object, so, like section.description, you'll use the md function to turn it into a string that fits in an HTML page
• Tests: nested.input.js is the only test fixture that generates a HTML output, so that's the place. Add a @classdesc tag to Klass and run it.

Shortcuts:

To quickly test changes to documentation.js, you can run ./bin/documentation.js instead of npm installing it. That will let you run, for instance

./bin/documentation.js serve test/fixture/html/nested.input.js

To update the test fixture, run

UPDATE=true npm test